Analysis of climatic and environmental variables associated with the occurrence of West Nile virus in Morocco

doi:10.1016/j.prevetmed.2013.02.011

Preventive Veterinary Medicine

Volume 110, Issues 3–4, 1 July 2013, Pages 549-553

https://doi.org/10.1016/j.prevetmed.2013.02.011 Get rights and content

Abstract

West Nile disease (WND) is one of the most widespread mosquito-borne infectious diseases in the World. In Morocco the first WND cases were reported in equines in 1996. After an apparent epidemiological silence, WND re-occurred in 2003 and in 2010, when the disease was reported in equines living in the central and north-western part of the country. Eco-climatic variables are known to influence the mosquito presence and abundance and, therefore, the probability of occurrence of mosquito-borne infections. The remote sensed values of Land Surface Temperature (LST), Normalised Difference Vegetation Index (NDVI) and rainfall registered from 2001 to 2010 were evaluated for a possible association with the occurrence of WND cases in 2003 and in 2010. In the zones where WND cases occurred, NDVI values recorded in 2003 and 2010, from June to November, were significantly higher than those registered during the same months in the rest of the decade. Rainfall data showed higher peaks in 2003 and 2010, when the number of days with extreme rainfall was significantly higher during 1–2 months before the occurrence of WND cases. Temperature does not seem to play an important role in Moroccan epidemiological conditions.

Introduction

West Nile virus (WNV) is a mosquito-transmitted Flavivirus belonging to the Japanese encephalitis antigenic complex of the Family Flaviviridae (Karabatos, 1985). It is transmitted primarily by the bite of mosquitoes being infected by feeding on viraemic birds (Kulasekera et al., 2001). Birds represent the vertebrate amplifying hosts (Komar et al., 2001), responsible for the virus maintenance in the environment and migratory species can be considered the main responsible for the geographical spread of the infection (Calistri et al., 2010). Humans, equines and other mammals are regarded as incidental or dead-end hosts. In the last decades an increasing number of cases of West Nile Disease (WND) was observed in equines and humans in the Mediterranean basin (Calistri et al., 2010).

Eco-climatic variables are known to influence the mosquito presence and abundance and, therefore, the probability of occurrence of mosquito-borne infections. Suitable climatic and environmental conditions influencing the abundance of mosquitoes and the density of susceptible hosts are the main factors influencing the occurrence of WND cases (Cantile et al., 2000). Several eco-climatic variables were found to be significantly associated with WND occurrence in many studies (Cantile et al., 2000, Mongoh et al., 2007, Ward, 2005, Ward, 2007, Ward, 2009). WNV was first detected in Morocco in 1996 (Tber Abdelhaq, 1996). After an apparent epidemiological silence, WND cases were notified in September and October 2003 in the province of Kénitra and, then, in August 2010, in the Rabat-Sale-Zemmour-Zaer and Tadla-Azilal regions, respectively in the north-western and central part of Morocco.

The aim of this study was to explore the differences between temperature, rainfall and vegetation index values recorded in 2010 and in 2003 and those recorded during years with no apparent WND cases. In particular, the values of Land Surface Temperature (LST), Normalised Difference Vegetation Index (NDVI) and precipitations from 2001 to 2010 were investigated. The following hypotheses were tested:

•
temperature was not a critical variable in Moroccan epidemiological conditions, given the latitude of the country,
•
higher NDVI values were associated with years of WND occurrence in equines (2003 and 2010), thus supporting the apparent multi-annual cyclicity observed in the occurrence of WND cases,
•
the occurrence of WND cases was linked to extreme rainfall of previous months.

Section snippets

Description of study areas

The province of Kénitra (in the region of Gharb-Chrarda-BéniHssen, in the north-western Morocco), which was involved in 2003 WNV transmission, is located in an extremely humid area that includes several wetlands and rice fields flooded by channels from the Sebou River. A natural bird reserve, Sidi Boughaba, is located 15 km southeast of Kénitra, along one of the migratory Europe-sub-Saharan routes (Schuffenecker et al., 2005). The climate is Mediterranean, with dry, hot summers and wet winters

Results

The geographical distribution of WND cases in Morocco and their 10 km radius areas are showed in Fig. 1. Table 1 reports the mean and standard deviation of NDVI values recorded from June to November during the decade 2001–2010 in each zone. For the “zone 2003” the NDVI values recorded in 2001–2002 and from 2005 to 2008 were significantly lower than those registered in 2003. For the “zone 2010” all NDVI values were significantly lower than 2010 with the exception of 2004. Table 2 shows the number

Discussion

WND epidemics are clearly linked to the abundance of vectors, in particular of mosquitoes of Culex genus (Komar, 2000). Variables affecting the distribution and the survival of mosquito populations, therefore, may influence the occurrence and the spread of the infection, given the availability of susceptible bird population as virus reservoirs. Eco-climatic variable such as temperature, rainfall, vegetation indices were proved to be associated with the occurrence of WND cases (Brownstein et

Conclusions

The preliminary analysis on the possible association between NDVI, temperature, rainfall and WND occurrence in Morocco shows a statistically significant difference between NDVI and precipitation values recorded during epidemic years (2003 and 2010) and those observed during the years with no WND cases. Temperature does not seem to play a crucial role in Moroccan epidemiological conditions. These results, although not exhaustive, might provide useful hints for further studies.

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Short communicationAnalysis of climatic and environmental variables associated with the occurrence of West Nile virus in Morocco

Abstract

Introduction

Section snippets

Description of study areas

Results

Discussion

Conclusions

Prev. Vet. Med.

Prev. Vet. Med.

Spatial analysis of West Nile virus: rapid risk assessment of an introduced vector-borne zoonosis

Vector Borne Zoo. Dis.

Epidemiology of West Nile in Europe and in the Mediterranean Basin

Open Virol. J.

Clinical and neuropathological features of West Nile virus equine encephalomyelitis in Italy

Equine Vet. J.

Effects of climate on West Nile Virus transmission risk used for public health decision-making in Quebec

Int. J. Health Geogr.

Remote sensing as a tool for mapping mosquito breeding habitats and associated health risk to assist control efforts and development plans: a case study in Wadi El Natroun, Egypt

J. Egypt Soc. Parasitol.

Analysis of vegetation and land cover dynamics in north-western Morocco during the last decade using MODIS NDVI time series data

Biogeosciences

International Catalogue of Arboviruses, Including Certain Other Viruses of Vertebrates

West Nile viral encephalitis

Rev. Sci. Tech.

Short communication
Analysis of climatic and environmental variables associated with the occurrence of West Nile virus in Morocco